Written in respondse to http://www.mmowned.com/forums/world-...-model-id.html
Go to Wowhead: YOU ARE NOW PREPARED and enter your NPC's name.
A window will appear with matches, select your NPC.
Confirm it's your NPC.
Find and remember the digits at the end of this url.
Download, install and launch WoWModelViewer - followed by going to "View" tab and selecting "View NPC".
Website: wowmodelviewer.org
Enter the digits you found on WoWhead.
There should only be 2 results at best, select the appropriate one.
You now have found the correct displayID for your NPC!
Questions and answers:
Why not skip WoWhead and search for the NPC name in the model viewer?
Because you're bound to run into multiple results, some which will bug out ingame.